A magnetic random access memory (MRAM) is a type of a resistance change memories. As techniques for writing data to an MRAM, there have been known magnetic field writing and spin-transfer torque writing. Among these techniques, the spin-transfer torque writing has advantages in higher integration, lower power consumption, and higher performance because of the property of a spin-transfer torque device that a spin injection current is smaller in an amount necessary for magnetization reversal as magnetic bodies become smaller in size.
A magnetic tunnel junction (MTJ) element of the spin-transfer torque has a stacked structure in which a nonmagnetic layer (an insulation film) is sandwiched between two ferromagnetic layers. The MTJ element stores therein digital data by a change in a magnetic resistance due to the spin-polarized tunneling. The MTJ element can be set in a low resistance state or a high resistance state in proportion to magnetization orientations of the two ferromagnetic layers. When the magnetization orientations of the two ferromagnetic layers are in a P (Parallel) state, the MTJ element is in the low resistance state, and when the magnetization orientations of the two ferromagnetic layers are in an AP (Anti Parallel) state, the MTJ element is in the high resistance state.
When writing data corresponding to the P state, it is necessary to supply a current necessary to reverse the state of the MTJ element from the AP state to the P state. When writing the AP state, it is necessary to supply a current necessary to reverse the state of the MTJ element from the P state to the AP state.
Conventionally, in an MRAM, two cell transistors are provided per one MTJ element so as to supply a sufficiently high write current to an MTJ element. One of a source and a drain of each cell transistor is connected to a lower end of the MTJ element, and the other one of the source and the drain is connected to a bit line via a via contact. Therefore, the layout area of a memory cell in the MRAM increases by as much as two via contacts corresponding to the two cell transistors. The memory cell size of the conventional MRAM is 16F2 (where F is a Feature Size).